Showing papers in "Soil Science Society of America Journal in 2013"

Tortuosity in Porous Media: A Critical Review

Abstract: The concept of tortuosity is used to characterize the structure of porous media, to estimate their electrical and hydraulic conductivity, and to study the travel time and length for tracer dispersion, but different types of tortuosity—geometric, hydraulic, electrical, and diffusive—have been used essentially interchangeably in the literature. Here, we critically review the tortuosity models developed empirically, analytically, and numerically for flow in both saturated and unsaturated porous media. We emphasize that the proposed tortuosity models are distinct and thus may not be used interchangeably. Given the variety of models that have been developed, and the sharp differences between some of them, no consensus has emerged unifying the models in a coherent way. Related treatments of tortuosity are found in the literature on porous catalysts. In such materials, nonlinear reactions ordinarily accompany transport, and the effective diffusivity within the pore space in the presence of the reactions is distinct from the one in their absence. Thus, because tortuosity may be defined as the ratio of the effective diffusivities in the bulk material and within the pore space, a careful treatment of tortuosity may need to distinguish between transport with and without reactions. This complication is ultimately relevant to soils as well, because bioremediation and biodegradation in soils are always accompanied by nonlinear reactions. Common models of tortuosity include both logarithmic functions and power laws. In many cases, the differences between the logarithmic and power-law phenomenologies are not great, but power laws can usually be reconciled with percolation concepts. Invoking percolation theory provides both insight into the origin of the power functions and a framework for understanding differences between tortuosity models.

State of the Art in Large‐Scale Soil Moisture Monitoring

Abstract: Soil moisture is an essential climate variable influencing land-atmosphere interactions, an essential hydrologic variable impacting rainfall-runoff processes, an essential ecological variable regulating net ecosystem exchange, and an essential agricultural variable constraining food security. Large-scale soil moisture monitoring has advanced in recent years, creating opportunities to transform scientific understanding of soil moisture and related processes. These advances are being driven by researchers from a broad range of disciplines, but this complicates collaboration and communication; and, for some applications, the science required to utilize large-scale soil moisture data is poorly developed. In this review, we describe the state of the art in large-scale soil moisture monitoring and identify some critical needs for research to optimize the use of increasingly available soil moisture data. We review representative examples of (i) emerging in situ and proximal sensing techniques, (ii) dedicated soil moisture remote sensing missions, (iii) soil moisture monitoring networks, and (iv) applications of large-scale soil moisture measurements. Significant near-term progress seems possible in the use of large-scale soil moisture data for drought monitoring. Assimilation of soil moisture data for meteorological or hydrologic forecasting also shows promise, but significant challenges related to spatial variability and model structures remain. Little progress has been made in the use of large-scale soil moisture observations within the context of ecological or agricultural modeling. Opportunities abound to advance the science and practice of large-scale soil moisture monitoring for the sake of improved Earth system monitoring, modeling, and forecasting.

High-Resolution 3-D Mapping of Soil Texture in Denmark

Abstract: Soil texture which is spatially variable in nature, is an important soil physical property that governs most physical, chemical, biological, and hydrological processes in soils. Detailed information on soil texture variability both in vertical and lateral dimensions is crucial for proper crop and land management and environmental studies, especially in Denmark where mechanized agriculture covers two thirds of the land area. We modeled the continuous depth function of texture distribution from 1958 Danish soil profiles (up to a 2-m depth) using equal-area quadratic splines and predicted clay, silt, fine sand, and coarse sand content at six standard soil depths of GlobalSoilMap project (0–5, 5–15, 15–30, 30–60, 60–100, and 100–200 cm) via regression rules using the Cubist data mining tool. Seventeen environmental variables were used as predictors and their strength of prediction was also calculated. For example, in the prediction of silt content at 0 to 5 cm depth, factors that registered a higher level of importance included the soil map scored (90%), landscape types (54%), and landuse (27%), while factors with lower scores were direct insolation (17%) and slope aspect (14%). Model validation (20% of the data selected randomly) showed a higher prediction performance in the upper depth intervals but increasing prediction error in the lower depth intervals (e.g., R² = 0.54, RMSE = 33.7 g kg⁻¹ for silt 0–5 cm and R² = 0.29, RMSE = 38.8 g kg⁻¹ from 100–200 cm). Danish soils have a high sand content (mean values for clay, silt, fine sand, and coarse sand content for 0- to 5-cm depth were 79, 84, 324, and 316 g kg⁻¹, respectively). Northern parts of the country have a higher content of fine sand compared to the rest of the study area, whereas in the western part of the country there was little clay but a high coarse sand content at all soil depths. The eastern and central parts of the country are rich in clay, but due to leaching, surface soils are clay eluviated with subsequent accumulation at lower depths. We found equal-area quadratic splines and regression rules to be promising tools for soil profile harmonization and spatial prediction of texture properties at national extentacross Denmark.

Replacing Fallow with Cover Crops in a Semiarid Soil: Effects on Soil Properties

Abstract: Replacement of fallow in crop–fallow systems with cover crops (CCs) may improve soil properties. We assessed whether replacing fallow in no-till winter wheat (Triticum aestivum L.)–fallow with winter and spring CCs for 5 yr reduced wind and water erosion, increased soil organic carbon (SOC), and improved soil physical properties on a Ulysses silt loam (fine-silty, mixed, superactive, mesic Aridic Haplustolls) in the semiarid central Great Plains. Winter triticale (×Triticosecale Wittm.), winter lentil (Lens culinaris Medik.), spring lentil, spring pea (Pisum sativum L. ssp.), and spring triticale CCs were compared with wheat–fallow and continuous wheat under no-till management. We also studied the effect of triticale haying on soil properties. Results indicate that spring triticale and spring lentil increased soil aggregate size distribution, while spring lentil reduced the wind erodible fraction by 1.6 times, indicating that CCs reduced the soil’s susceptibility to wind erosion. Cover crops also increased wet aggregate stability and reduced runoff loss of sediment, total P, and NO₃–N. After 5 yr, winter and spring triticale increased SOC pool by 2.8 Mg ha–¹ and spring lentil increased SOC pool by 2.4 Mg ha–¹ in the 0- to 7.5-cm depth compared with fallow. Triticale haying compared with no haying for 5 yr did not affect soil properties. Nine months after termination, CCs had, however, no effects on soil properties, suggesting that CC benefits are short lived in this climate. Overall, CCs, grown in each fallow phase in no-till, can reduce soil erosion and improve soil aggregation in this semiarid climate.

Straw Mulching Effect on Splash Erosion, Runoff, and Sediment Yield from Eroded Plots

Abstract: Land surface cover affects runoff generation and soil loss processes. Splash erosion occurs by the impact of raindrops on the soil surface. As a result of this process, raindrops detach soil particles, destroy soil structure, and finally, increase runoff and erosion. Amendments improve and reinforce soil aggregates and some physical barriers against raindrop impacts and runoff. The present research was an attempt to determine the efficiency of straw mulch applied at a rate of 0.5 g m⁻² in changing the runoff commencement time, runoff amount, splash erosion, and sediment yield from eroded mid-sized plots at different rainfall intensities under laboratory conditions. The research was conducted on a sandy loam soil taken from summer rangeland in the Alborz Mountains, northern Iran, with simulated rainfall intensities of 30, 50, 70, and 90 mm h⁻¹ and a slope of 30% in three replicates. The collected data from 36 splash cups showed that the straw mulch played an effective role in reducing the splash erosion in both up and down directions, with the maximum reduction occurring at a rainfall intensity of 70 mm h⁻¹. The results of the research also showed that the straw mulch had a significant effect in changing runoff and soil erosion characteristics at a confidence level of 99%. The maximum increase in runoff commencement time (110.10%) was observed for the rainfall intensity of 90 mm h⁻¹. The runoff coefficient had a maximum reduction at rainfall intensities of 30 and 90 mm h⁻¹. The maximum decrease in sediment yield (63.24%) also occurred at the rainfall intensity of 90 mm h⁻¹.

Interpretation of Microbial Soil Indicators as a Function of Crop Yield and Organic Carbon

Abstract: An interpretative framework for microbial biomass C (MBC), basal respiration, and the activity of soil enzymes cellulase, β-glucosidase, arylsulfatase, and acid phosphatase was developed for the clayey Oxisols of the Brazilian Cerrado. Soil samples (0–10-cm depth) were collected from 24 treatments from three long-term experiments and analyzed to determine their microbial attributes and soil organic C (SOC). These treatments presented a large range of Mehlich-extractable P and cumulative corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] yields. The critical levels for the microbial indicators were defined based on criteria similar to those used in soil nutrient calibration tests. The microbial indicators were interpreted as a function of the relative cumulative yields (RCYs) of corn and soybean and the SOC using linear regression models. Adequacy classes for each microbial indicator as a function of the RCY and SOC were established based on the following criteria: ≤40%: low; 41 to 80%: moderate; and >80%: adequate. The critical levels equivalent to 80% of the RCY for MBC, basal respiration, cellulase, β-glucosidase, acid phosphatase, and arylsulfatase were: 375 mg C kg⁻¹, 90 mg CO₂–C kg⁻¹, 105 mg glucose kg⁻¹ d⁻¹, 115 mg p-nitrophenol kg⁻¹ h⁻¹, 1160 mg p-nitrophenol kg⁻¹ h⁻¹, and 90 mg p-nitrophenol kg⁻¹ h⁻¹, respectively. Similar critical levels were obtained when SOC was used as the interpretation criterion. The interpretation tables provided in this study establish, for the first time, reference values for the soil microbial indicators based on crop yields and constitute a first approximation. Their applicability to other conditions must be evaluated.

Effects of Mulch Cover Rate on Interrill Erosion Processes and the Size Selectivity of Eroded Sediment on Steep Slopes

Abstract: Mulching with vegetative residue is an effective soil conservation practice. A better understanding of sediment characteristics associated with various mulch rates would improve the use of this practice for soil conservation. An experiment was conducted to evaluate the effects of straw mulch on runoff, erosion, and the particle-size distribution (PSD) of eroded sediment. Straw mulch rates of 0, 15, 30, 50, 70, and 90% cover were tested using simulated rainfall. The effective PSD of sediment (undispersed) was compared with equivalent measurements of the same samples after dispersion (ultimate PSD) to investigate the detachment and transport mechanisms involved in sediment mobilization. The maximum stream occurred at a different time from the peak sediment concentration during rainstorms under low mulch rates, which indicated the predominance of supply-limited conditions. However, at higher mulch rates the erosion processes were typical of a transport-limited sediment regime. The ratio of the sediment transported as primary clay to the soil matrix clay content was always less than 1, meaning that most of the clay was eroded in the form of aggregates. Transport selectivity was reflected by the silt enrichment, and silt-sized particles were transported mainly as primary particles since their effective-ultimate ratio was close to 1. The enrichment ratios for the sand-sized fractions decreased from 0.98 to 0.38 with increased mulch rates, and effective-ultimate ratios for sand-sized particles were always greater than 1, indicating that most of these particles were predominantly aggregates of finer particles, especially at high mulch rates. The findings reported in this study have important implications for the assessment and modeling of interrill erosion processes.

Changes in Relative Gas Diffusivity Explain Soil Nitrous Oxide Flux Dynamics

Abstract: Soil Sci. Soc. Am. J. doi:10.2136/sssaj2013.04.0141 Received 13 Apr. 2013. *Corresponding author (nimlesh.nimlesh@lincolnuni.ac.nz). © Soil Science Society of America, 5585 Guilford Rd., Madison WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher. Changes in Relative Gas Diffusivity Explain Soil Nitrous Oxide Flux Dynamics Soil Physics

Afforestation Effects on Soil Carbon Storage in the United States: A Synthesis

Abstract: Afforestation (tree establishment on nonforested land) is a management option for increasing terrestrial C sequestration and mitigating rising atmospheric carbon dioxide because, compared to nonforested land uses, afforestation increases C storage in aboveground pools. However, because terrestrial ecosystems typically store most of their C in soils, afforestation impacts on soil organic carbon (SOC) storage are critical components of ecosystem C budgets. We applied synthesis methods to identify the magnitude and drivers of afforestation impacts on SOC, and the temporal and vertical distributions of SOC change during afforestation in the United States. Meta-analysis of 39 papers from 1957 to 2010 indicated that previous land use drives afforestation impacts on SOC in mineral soils (overall average = +21%), but mined and other industrial lands (+173%) and wildlands (+31%) were the only groups that specifically showed categorically significant increases. Temporal patterns of SOC increase were statistically significant on former industrial and agricultural lands (assessed by continuous meta-analysis), and suggested that meaningful SOC increases require ≥15 and 30 yr of afforestation, respectively. Meta-analysis of ¹³C data demonstrated the greatest SOC changes occur at the surface soil of the profile, although partial replacement of C stocks derived from previous land uses was frequently detectable below 1 m. A geospatial analysis of 409 profiles from the National Soil Carbon Network database supported ¹³C meta-analysis results, indicating that transition from cultivation to forest increased A horizon SOC by 32%. In sum, our findings demonstrate that afforestation has significant, positive effects on SOC sequestration in the United States, although these effects require decades to manifest and primarily occur in the uppermost (and perhaps most vulnerable) portion of the mineral soil profile.

Cover Crop Effects on Nitrous Oxide Emissions: Role of Mineralizable Carbon

Abstract: Nitrous oxide (N₂O) emission from denitrification in agricultural soils often increases with N fertilizer and soil nitrate (NO₃) concentrations. Overwintering cover crops in cereal rotations can decrease soil NO₃ concentrations and may decrease N₂O emissions. However, mineralizable C availability can be a more important control on N₂O emission than NO₃ concentration in fertilized soils, and cover crop residue provides mineralizable C input. We measured the effect of a winter rye (Secale cereale L.) cover crop on soil N₂O emissions from a maize (Zea mays L.) cropping system treated with banded N fertilizer at three rates (0, 135, and 225 kg N ha–¹) in Iowa. In addition, we conducted laboratory incubations to determine if potential N₂O emissions were limited by mineralizable C or NO₃ at these N rates. The rye cover crop decreased soil NO₃ concentrations at all N rates. Although the cover crop decreased N₂O emissions when no N fertilizer was applied, it increased N₂O emissions at an N rate near the economic optimum. In laboratory incubations, N₂O emissions from soils from fertilizer bands did not increase with added NO₃, but did increase with added glucose. These results show that mineralizable C availability can control N₂O emissions, indicating that C from cover crop residue increased N₂O emissions from fertilizer band soils in the field. Mineralizable C availability should be considered in future evaluations of cover crop effects on N₂O emissions, especially as cover crops are evaluated as a strategy to mitigate agricultural greenhouse gas emissions.

Diffuse-Reflectance Fourier-Transform Mid-Infrared Spectroscopy as a Method of Characterizing Changes in Soil Organic Matter

Abstract: Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not impy recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer. Soil Sci. Soc. Am. J. 77:1591–1600 doi:10.2136/sssaj2013.04.0131 Supplemental material is available online for this article. Received 11 Apr. 2013. *Corresponding author (francisco.calderon@ars.usda.gov). © Soil Science Society of America, 5585 Guilford Rd., Madison WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher. Diffuse-Reflectance Fourier-Transform Mid-Infrared Spectroscopy as a Method of Characterizing Changes in Soil Organic Matter Soil Biology & Biochemistry

Soil Heterogeneity and Soil Fertility Gradients in Smallholder Farms of the East African Highlands

Abstract: Heterogeneity in soil fertility in these smallholder systems is caused by both inherent soil-landscape and human-induced variability across farms differing in resources and practices. Interventions to address the problem of poor soil fertility in Africa must be designed to target such diversity and spatially heterogeneity. Data on soil management and soil fertility from six districts in Kenya and Uganda were gathered to understand the determinants of soil heterogeneity within farms. Analysis of the variance of soil fertility indicators across 250 randomly selected farms (i.e., 2607 fields), using a mixed model that considered site, sampling frame, farm type, and field as random terms, revealed that the variation in soil organic C (6.5–27.7 g kg-1), total N (0.6–3.0 g kg-1), and available P (0.9–27 mg kg-1) was mostly related to differences in the inherent properties of the soils across sites (50 to 60% of total variance). Exchangeable K+ (0.1–1.1 cmol(+) kg-1), Ca2+ (1.5–14.5 cmol(+) kg-1), Mg2+ (0.6–3.7 cmol(+) kg-1), and pH (5.1–6.9) exhibited larger residual variability associated with field-to-field differences within farms (30 to 50%). Soil fertility indicators decreased significantly with increasing distance from the homesteads. When this variable was included in the model, the unexplained residual variances—associated with soil heterogeneity within farms—were 38% for soil C; 32% for total N; 49% for available P; 56, 49, and 38% for exchangeable K+, Ca2+ and Mg2+, respectively; and 49% for the pH. In allocating nutrient resources, farmers prioritized fields they perceived as most fertile, reinforcing soil heterogeneity. Categorization of fields within a farm with respect to distance from the homestead, and soil fertility classes as perceived by farmers, were identified as entry points to target soil fertility recommendations to easily recognizable, distinct entities.

Percolation Theory Generates a Physically Based Description of Tortuosity in Saturated and Unsaturated Porous Media

Abstract: Tortuosity is a property of porous media that is invoked and used in the literature on hydrology, soil science, physics, and engineering It has been defined in a variety of ways, one of which is a purely geometrical concept In this study, we focused on the geometrical tortuosity and developed a model based on percolation theory and the finite-size scaling approach Our result, developed for porous media of any saturation, expresses the tortuosity as a power-law function of the water content, the critical water content, and the system size The model parameters include the fractal dimension of either the backbone (the flow-carrying part of porous media) or the optimal path (for crossing the system between two opposite faces), which have clear physical meaning The results may be combined with power laws, which percolation theory provides for the hydraulic or electrical conductivity, to develop the appropriate form that expresses the connectivity as a function of the water content of a pore space Comparison with numerical simulations and experiments reported in the literature indicated that the model estimates the tortuosity accurately Our model and results leads us to conclude that pore connectivity and tortuosity should be treated as two distinct properties and that significant uncertainty in results for the saturation dependence of the hydraulic conductivity is traceable to the confusion that mixes the two as a single concept

Rebuilding Soils on Mined Land for Native Forests in Appalachia

Abstract: The eastern U.S. Appalachian region supports the world’s most extensive temperate forests, but surface mining for coal has caused forest loss. New reclamation methods are being employed with the intent of restoring native forest on Appalachian mined lands. Mine soil construction is essential to the reforestation process. Here, we review scientific literature concerning selection of mining materials for mine soil construction where forest ecosystem restoration is the reclamation goal. Successful establishment and productive growth of native Appalachian trees has been documented on mine soils with coarse fragment contents as great as 60% but with low soluble salt levels and slightly to moderately acidic pHs, properties characteristic of the region’s native soils. Native tree productivity on some Appalachian mined lands where weathered rock spoils were used to reconstruct soils was found comparable to productivity on native forest sites. Weathered rock spoils, however, are lower in bioavailable N and P than native Appalachian soils and they lack live seed banks which native soils contain. The body of scientific research suggests use of salvaged native soils for mine soil construction when forest ecosystem restoration is the reclamation goal, and that weathered rock spoils are generally superior to unweathered rock spoils when constructing mine soils for this purpose.

Particle Interaction Forces Induce Soil Particle Transport during Rainfall

Abstract: Soil particle transport causes serious environmental problems, and particle interaction forces will be the controlling force for soil transport during rainfall. However, few reports can be found in literature concerning the quantitative evaluation of particle interaction forces on soil particle transport. In this study, purple soil was adopted to study soil particle transport under different force strength conditions of particle interaction during rainfall simulation with precipitation intensity of 150 mm h–¹ for 110 min. We first analyzed soil particle interaction forces quantitatively under different KNO₃ and Ca(NO₃)₂ concentrations (0.0001, 0.001, 0.005, 0.01, 0.05, 0.1, and 1 mol L–¹) which adjusted surface potential of soil particle from –323 to –50 mV. The results indicated that the net Derjaguin–Landau–Verwey–Overbeek (DLVO) force could not solely cause soil particle transport during rainfall. The hydration repulsive force, which was much stronger than the DLVO force, played a crucial role in soil particle transport. During rainfall, soil particle in aggregate was separated to a distance of 1.5 to 2 nm by the strong hydration repulsive force (swelling process). Then aggregate was thoroughly broken by the electrostatic repulsive force. The hydration repulsive force dominated aggregate swelling, and the electrostatic force determined the transport intensity. We found that, at surface potentials lower than –100 mV, soil transport did not occur even precipitation intensity reaching 150 mm h–¹; and –200 mV of surface potential was the critical potential for soil particle transport. We conclude that the surface hydration force and electrostatic repulsive force were the controlling force for soil particle transport during rainfall.

Comparing Methods to Determine Hydraulic Conductivities on Stony Soils

Abstract: Determination of the field-saturated hydraulic conductivity (Kfs) can result in very high variability due to soil heterogeneity, the measurement method, the number of replications, and the Kfs calculation method used. Especially for dryland soils, stoniness can influence infiltration rates significantly. To identify this variability as well as its source, six widely used measurement methods were compared: single-ring (SR) and double-ring (DR) infiltrometers, the constant head well infiltrometer (CH), the inverse auger hole method (IA), the tension infiltrometer (TI), and the rainfall simulator (RFS). The six methods were applied at three locations in a semiarid part of Chile that showed moderate (15%) to high (55%) stoniness. Additionally, Kfs variations due to different calculation techniques for the same measurement method were thoroughly investigated. Results showed that different calculation techniques sometimes gave significantly different estimates of Kfs when using the same data set, and those relative differences were conserved among measurement locations. The borehole methods (IA and CH) showed high discard rates due to stoniness, making these methods less appropriate. The SR and DR methods gave considerably higher Kfs estimates, while the RFS and TI proved good candidates as reference methods for stony soils, with low failure rates and coefficients of variation.

Effects of Biochar Amendment on Soil Thermal Conductivity, Reflectance, and Temperature

Abstract: Little information is available regarding the effects of biochar amendment on soil thermal properties and soil temperature, especially under field conditions. The possible changes in soil thermal conductivity, surface reflectance, and temperature resulting from biochar addition might affect other biophysical–chemical processes in the soil. We examined the effects of biochar amendment on the thermal conductivity, surface reflectance, and temperature of soil. Soil thermal conductivity, reflectance, and temperature at a 5-cm depth were measured and monitored in an experimental field using a portable soil thermal property analyzer, a portable full-range spectrometer, and temperature and water potential probes. The field was located in the North China Plain and had been cropped in a winter wheat (Triticum aestivum L.)–maize (Zea mays L.) system for 5 yr. With biochar amendment, soil thermal conductivity was decreased significantly by 3.48 and 7.49% with 4.5 (B4.5) and 9.0 t ha⁻¹ yr⁻¹ (B9.0) of biochar addition, respectively, which was consistent with a decrease in soil bulk density. Soil water potential slightly increased under B4.5 treatment but decreased under B9.0 treatment relative to the control. Reflectance increased in the near-ultraviolet and blue-light wavelengths (350–513 nm) and decreased in the infrared wavelength range (520–2350 nm) with biochar amendment. Biochar amendment reduced diurnal soil-temperature fluctuations on both daily and seasonal scales, although the annual average daily soil temperatures at a 5-cm depth showed no significant difference among treatments. Comparison with the control showed that biochar treatment moderated soil temperature extremes, lowering the temperature when soil temperature was high and raising it when soil temperature was low. This moderating capability was mostly within ±0.4 and ±0.8°C for mean daily temperature and mean diurnal temperature of soil, respectively. The effect of biochar amendment on soil temperatures can be explained by the combined action of changes in soil thermal conductivity and reflectance.

Revealing Soil Structure and Functional Macroporosity along a Clay Gradient Using X-ray Computed Tomography

Abstract: The influence of clay content in soil-pore structure development and the relative importance of macroporosity in governing convective fluid flow are two key challenges toward better understanding and quantifying soil ecosystem functions. In this study, soil physical measurements (soil-water retention and air permeability) and x-ray computed tomography (CT) scanning were combined and used from two scales on intact soil columns (100 and 580 cm3). The columns were sampled along a natural clay gradient at six locations (L1, L2, L3, L4, L5 and L6 with 0.11, 0.16, 0.21, 0.32, 0.38 and 0.46 kg kg−1 clay content, respectively) at a field site in Lerbjerg, Denmark. The water-holding capacity of soils markedly increased with increasing soil clay content, while significantly higher air permeability was observed for the L1 to L3 soils than for the L4 to L6 soils. Higher air permeability values observed for 580- than 100-cm3 soil columns implied a scale effect and relatively greater importance of macropores in convective fluid flow at larger scale. Supporting this, x-ray CT showed that both interaggregate pores and biopores (pores formed by earthworms and plant roots) were present at L1 to L3 in decreasing order, whereas only interaggregate pores were observed at L4 to L6. Macroporosity inferred from x-ray CT to quantify pores >1 mm decreased from 2.9 to 0.1% from L1 to L6. A progressive improvement was observed in the linear relationship (R2 increasing 0.50–0.95) of air permeability with total air-filled porosity, CT-inferred macroporosity, and CT-inferred limiting macroporosity (minimum macroporosity for any quarter of soil column). The findings of this study show the immense potential in linking x-ray CT-derived soil-pore parameters with classical soil physical measurements for quantifying soil architecture and functions.

Phosphorus Speciation in Riparian Soils: A Phosphorus-31 Nuclear Magnetic Resonance Spectroscopy and Enzyme Hydrolysis Study

Abstract: In the Lake Champlain Basin, phosphorus (P) loading from streambank erosion and cropland are both important P sources, and a better understanding of the factors affecting riparian P loss is needed to help prioritize riparian restoration efforts. We utilized solution phosphorus-31 nuclear magnetic resonance (NMR) spectroscopy and an enzyme hydrolysis method to characterize P and assess bioavailability in 14 commonly mapped riparian soils from northwestern Vermont. Surface horizons were sampled from distinct series at two riparian restoration sites to capture a range of soil properties. Samples were extracted with sodium hydroxide–ethylenediaminetetra-acetic acid (NaOH-EDTA) and analyzed by solution ³¹P NMR to speciate and quantify P compounds, and commercially available phosphatase enzymes were used to fractionate water-extractable molybdate unreactive P (MUP) into labile orthophosphate monoesters and orthophosphate diesters. Phosphorus extracted by NaOH-EDTA ranged from 74 to 510 mg P kg⁻¹ (representing 14.2 to 31.9% of total soil P), of which 58 ± 13% was identified as organic P. Phosphorus compounds identified in all samples included myo-inositol hexakisphosphate (myo-IHP), scyllo-IHP, neo-IHP, chir-IHP, glycerophosphate, glucose 6-phosphate, mononucleotides, choline phosphate, glucose 1-phosphate, DNA, pyrophosphate, and orthophosphate. Orthophosphate monoesters accounted for 53.7 ± 12.3% of total NaOH-EDTA extractable P and 93 ± 3% of the NaOH-EDTA organic P, indicating the importance of organic P in these soils. Stereoisomers of IHP accounted for 29 ± 7% of NaOH-EDTA extractable Pₒ. For the water extractions, 78 ± 13% of total P was MUP, of which 18 ± 6% was labile orthophosphate monoesters and 31 ± 15% was orthophosphate diesters. Results suggest that analytical indices of riparian P loss potential should consider both organic and inorganic P.

Identifying Six Types of Soil Shrinkage Curves from a Large Set of Experimental Data

Abstract: A typical soil shrinkage curve is S-shaped and composed of four phases termed structural, proportional, residual, and zero shrinkage. However, many studies have not found all four soil shrinkage phases despite investigating the full spectrum of soil moisture content. The objectives of this paper were to determine different soil shrinkage types based on the presence of shrinkage phases and to define relationships between the parameters of different shrinkage types and soil properties. A total of 270 sets of shrinkage data were collected from published (N = 245) and our unpublished work (N = 25), covering a wide range of soil types, sample sizes, and measurement methods. According to the presence of different shrinkage phases, six types of soil shrinkage curves were classified using the shrinkage model proposed by Peng and Horn (2005). Soil shrinkage types generally depended on soil structure, but not on the measurement method. The coefficient of linear extensibility (COLE) had a positive relation with saturated soil bulk density (r = 0.50, P < 0.001), clay content (r = 0.20, P < 0.05), and soil organic carbon (SOC) content (r = 0.46, P < 0.001). This paper is the first to propose six soil shrinkage types that will improve our understanding of the relationship between soil structure and soil water content.

What's wrong with soil physics?

Abstract: Soil physics has a dual identity—it is both a branch of physics and a branch of soil science—but its legitimacy as a science depends on its claim to be physics; this implies a self-consistent structure of definitions and concepts underlying the equations we actually use. Upon examining some of our core concepts—specifically those relating the water retention curve to the pore size distribution, the unsaturated hydraulic conductivity relationship, and the convection–dispersion model—we find that all three are built on the notion that soil is composed of bundles of capillary tubes. This underlying conceptual model lacks both self-consistency (threatening our claim to be a legitimate science) and a firm connection to reality (threatening our ability to reason and predict successfully). We argue that many of our struggles during the last decades are artifacts of building from the flawed conceptual model of soil as a capillary bundle. We propose in its place a pore network concept, which can be applied using the mathematics of percolation theory. We must build on a sound and self-consistent conceptual model, in teaching, research, and application, so soil physics can be firmly based in both soils and physics, and meet society’s many challenges in food production, hydrology, water quality, bio-energy, and climate change.

Long‐Term Tillage Impacts on Soil Aggregation and Carbon Dynamics under Wheat‐Fallow in the Central Great Plains

Abstract: Soil Sci. Soc. Am. J. 77:594–605 doi:10.2136/sssaj2012.0125 Received 12 Apr. 2012. *Corresponding author (maysoon.mikha@ars.usda.gov). Mention of commercial products and organization in this paper is solely to provide specific information. It does not constitute endorsement by USDA-ARS over other products and organization not mentioned. The US Department of Agriculture, Agricultural Research Service, is an equal opportunity–affirmative action employer and all agency services are available without discrimination.

Gas Diffusion, Non-Darcy Air Permeability, and Computed Tomography Images of a Clay Subsoil Affected by Compaction

Abstract: Soil productivity and other soil functions are dependent on processes in the untilled subsoil. Undisturbed soil cores were collected at the 0.3- to 0.4-m depth from a heavy clay soil in Finland subjected to a single heavy traffic event by agricultural machinery three decades before sampling. Untrafficked control plots were used as a reference. Computed tomography (CT) scanning was performed on soil cores at a field-sampled field capacity water content. Gas diffusion and air permeability were measured when the soil cores were drained to −1000 hPa matric potential (air permeability also at −100 and −300 hPa). The air-filled pore space was measured with an air pycnometer and also calculated from mass balance and CT data. Gas diffusion and air permeability were also measured on a straight model tube and on autoclaved aerated concrete. The compaction treatment had not influenced soil total porosity but had significantly lowered the volume fraction of air-filled macropores at the matric potentials investigated. The compacted soil displayed significantly lower air permeability, while gas diffusivity was not affected. Our analyses indicate that this was due to a compaction-induced reduction in the cross-sectional area of vertical, arterial macropores and in the volume of marginal pores branching from these vertical pores. We observed non-Darcian air flow during air permeability measurements and thus suggest the use of a nonlinear regression technique based on measurements at several pressure differences to arrive at true Darcian air permeability. The tests on artificial materials supported the conclusions that the dominating pores in this clayey subsoil are nearly straight, vertical macropores.

Relationships between Biological and Thermal Indices of Soil Organic Matter Stability Differ with Soil Organic Carbon Level

Abstract: Soil Sci. Soc. Am. J. 77:2020–2028 doi:10.2136/sssaj2013.02.0081 Supplementary material is available online for this article. Received 28 Feb. 2013. *Corresponding author (peltre@life.ku.dk). © Soil Science Society of America, 5585 Guilford Rd., Madison WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher. Relationships between Biological and Thermal Indices of Soil Organic Matter Stability Differ with Soil Organic Carbon Level Soil Biology & Biochemistry

Winter Nitrate Leaching under Different Tillage and Winter Cover Crop Management Practices

Abstract: The potential for nitrate (NO₃–) to leach is enhanced following cultivation of pastoral land, due to rapid mineralization of labile soil organic matter (SOM). In a 7-yr field trial in New Zealand, we examined the impacts of tillage intensity [intensive (plowing to 20 cm), minimum, or no-tillage] and winter cover crops (forage rape) on NO₃– leaching following cultivation of permanent (sheep [Ovis aries]-grazed) pasture to grow arable crops (the rotation included barley [Hordeum vulgare L.], wheat [Triticum aestivum L.], and pea [Pisum sativum L.]). Permanent pasture and permanent fallow (maintained plant-free using herbicides, i.e., not cultivated or fertilized) treatments were included as controls. Losses of NO₃––N were calculated from soil solution NO₃––N concentrations (measured in ceramic suction cups installed at 600 mm) and drainage volumes. Cumulative NO₃––N leached over 7 yr ranged from 20 to 428 kg N ha– ¹, with least N lost under pasture. Residual soil mineral N in autumn accounted for ∼30% of the variability in leaching. Nitrate leaching under arable crops generally increased rapidly as winter rainfall (range 78–352 mm yr– ¹) increased. Winter cover crops were effective in reducing NO₃––N leaching losses, particularly in drier winters when about 50% less N leached where cover crops were grown. On average, annual leaching was only 10 to 18 kg N ha– ¹ in the presence of cover crops. Tillage had relatively little influence on leaching, though use of minimum tillage for autumn cultivation resulted in significantly (P < 0.001) less NO₃––N leaching than either intensive or no tillage. Largest leaching losses were recorded in the unfertilized, permanent fallow where there was no plant sink for NO₃–N derived from SOM mineralization. Growing a crop during the winter period, combined with good N management practices to minimize pre-winter soil mineral N, provides the best option to keep NO₃– leaching within the acceptable range for arable cropping in New Zealand.

Naturally Occurring Asbestos: Potential for Human Exposure, Southern Nevada, USA

Abstract: Amphibole asbestos minerals are known human carcinogens, and many regulations have been developed to limit occupational exposure. These minerals can also occur in the natural environment, where they may be more difficult to control. We applied a diverse set of analytical methods including scanning electron microscopy/energy dispersive spectroscopy, electron probe analysis, x-ray diffraction, and field-emission scanning electron microscopy to rock, soil, and dust samples and to particles attached to clothing samples and cars. We found naturally occurring fibrous actinolite, a regulated amphibole asbestos mineral, in rock, soil, and dust that can be transported by wind, water, cars, or on clothing after outdoor recreational activities. Sources of these fibrous amphiboles are several plutons in southern Nevada and Arizona and alluvial fans emanating from asbestos-containing bedrock. The morphology of the amphibole fibers is similar to amphibole fibers found in the USEPA Superfund site at Libby, MT. We found that the morphometry of the fibrous particles in the study area did not substantially change when the original bedrock weathered into soil, and particles were eroded and transported through wind and/or water and finally settled and accumulated on natural or other surfaces. Because large populations in Boulder City, Henderson, and Las Vegas are located only a few kilometers, sometimes even only a few tens of meters, downwind from the sources, and because most of the particles are transported in suspension after they are emitted, potentially large populations in Boulder City, Henderson, and perhaps Las Vegas could be exposed. This study demonstrates a potential public health risk to several large population areas.

Modeling Selenite Adsorption Envelopes on Oxides, Clay Minerals, and Soils using the Triple Layer Model

Abstract: Selenite adsorption behavior was investigated on amorphous Al and Fe oxides, clay minerals: kaolinite, montmorillonite, and illite, and 45 surface and subsurface soil samples from the southwestern and midwestern regions of the United States as a function of solution pH. Selenite adsorption decreased with increasing solution pH. The triple layer model, a chemical surface complexation model, was able to describe selenite adsorption as a function of solution pH by simultaneously optimizing both inner-sphere and outer-sphere selenite surface complexation constants. The fit of the triple layer model to selenite adsorption by soils was much improved over that obtained previously by optimizing solely an inner-sphere selenite surface complexation constant and the protonation constant in the constant capacitance model. In this previous application, the deprotonation constant had been neglected; thereby, preventing the reactive surface hydroxyl group from deprotonating; a chemically unrealistic situation. The selenite surface speciation predicted using the triple layer model was in agreement with that obtained for other strongly adsorbing anions such as molybdate. Direct spectroscopic investigations of selenite surface configuration are needed to corroborate the species predicted by the modeling approach.